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Application of high resolution lithium-drift germanium gamma-ray spectrometers to high energy gamma-rays
NUCLEAR
INSTRUMENTS
AND
METHODS
26
(1964)
183--186;
©
NORTH-HOLLAND
PUBLISHING
CO.
APPLICATION OF HIGH RESOLUTION LITHIUM-DRIFT GERMANIUM GAMMA-RAY SPECTROMETERS TO HIGH ENERGY GAMMA-RAYS G. T. E \ V A N a n d A. J. T A V E N D A L E
Chalk River Nuclear Laboratories, Atomic Energy o/Canada Limited Received 2 J a n u a r y 1964
The process is illustrated schematically in the inset to fig. 1 below. At a y-ray energy of 7.64 MeV we have achieved a resolution of 9.8 keV, corresponding to a y-ray energy resolution of 0.13 %. This is better
In a recent publication 1) we discussed briefly the use of germanium lithium-drift p-i-n diodes as high resolution y-ray spectrometers. W e report here the application of these detectors to the study of high DOUBLE
ESCAPE
Y-2614
1~92 key
-Z
3 keV
z io
7-26L4
ke~
w o_
o
,o'
iOZ[ I 1400
i
_
J
1600
..........
18100 .....
22:00
2000
2400
26100
ENERGY (keV) Fig. 1. H i g h energy region of g a m m a - r a y s p e c t r u m of Th(B -F C + C") observed with a l i t h i u m - d r i f t g e r m a n i u m detector. The detector was operated a t liquid n i t r o g e n t e m p e r a t u r e with a bias of 200 V. The spectrum of the 2614 keV g a m m a - r a y shows the intense double-escape peak, the weak single-escape peak, the full energy peak and the C o m p t o n d i s t r i b u t i o n . Tile origin of the double-escape peak is shown in the inset. I t is this peak t h a t we use in s t u d y i n g high energy gamma-rays.
energy y-rays. For this purpose we make use of the absorption of the y-rays by pair production and the detection of the total energy of the electron-positron pair in the crystal. This gives a peak at ( E - - 2roDc2) which for high energy y-rays is much more intense than the background due to Compton scattering. * N.R.C. Postdoctoral Felllow.
than has so far been obtained with either magnetic pair spectrometers or Compton spectrometers. The high resolution, combined with the simplicity of the detector, will make possible many new experii) A. J. T a v e n d a l e and G. T. Ewan, Nucl. Instr. and Meth., 25 1963) 185. iX. J. Tavendale, Proceedings of I n t e r n a t i o n a l S y m p o s i u m on Nuclear Electronics, Paris 1963, in press. 183
184
G. T. E W A N A N D A. J. T A V E N D A L E
merits with reaction and neutron capture ?-rays. To illustrate the type of spectrum obtained from a high energy ?-ray, we showin fig. 1 the spectrum observed from a radio-active source of Th (B + C + C"), using a detector 19 mm in diameter and with a depletion depth of 3.5 mm. The detector was operated at 77°K with a bias of 200 volts. The 2.614 MeV ?-ray has a peak corresponding to the full
k
5.297
from the double-escape peak to higher energies. For the crystal used here, this effect reduces the calculated intensity of the double-escape peak by a factor of ~ 0.6. We have used a detector 19 mm in diameter with a depletion depth of 3.5 mm to study ?-rays from reactions produced by the Chalk River Tandem accelerator, and neutron capture gamma-rays at MeV
N 14 ( d , p ) N '5
GAMMA-RAY MeV
I000
(3/2 -)
s 3 z l - -
1/2+ /' J bJ z ;;E
5.269 MeV
800
GAMMA-RAY
I
o
i a: w a_
600
8.5 keV
I o
_
:
I
I!
400
!
~• . . . .
i
5POO
"..%
• • . •. ,~ :.:..'~." :~-~-.,-~.4,,--;~..
v
I
,/~
a 15
Tantalum nitride target 4 MeV
deuterons
Germanium lithium drift p-i-n diode at liquid nitrogen temperature 19 mm diameter z 3-5 mm depletion depth
J
Gamma rays /Both annihilation ~ q ~ n t a escape
I
I
5200 T-RAY
5300 ENERGY
(keY)
Fig. 2. Region of g a m m a - r a y spectrum observed in the reaction N14(d, p) N 15 produced by 4 MeV deuterons. A schematic d i a g r a m of the e x p e r i m e n t a l a r r a n g e m e n t is shown a t the right. The g a m m a ray energy resolution is 0.16%.
energy, a Compton distribution, a weak singleescape peak and an intense peak due to pair production in which the electron and positron lose all their energy in the crystal, and the two annihilation quanta escape. It is this "double escape p e a k " which makes the detector very useful for studying high energy y-rays. The height of the peak is 8 times the height of the Compton distribution. For higher energy y-rays the pair production cross-section increases and the Compton cross-section decreases, so that the ratio of peak height to Compton distribution will improve. The resolution (fwhm) on the "double escape peak" is 7.3 keV and the measured intrinsic efficiency is ~ 0.3 %. This efficiency is less than would be calculated from the pair production cross-section because absorption or Compton scattering of the annihilation quanta will remove counts
the N R X reactor. In fig. 2 we show a small region of the N14(d,p)N15 y-ray spectrum. The doublet at 5.27 and 5.30 MeV is clearly resolved. Because of the relatively small depletion depth there is a tail on the low energy side of the lines due to escape of a large fraction of the electrons from the crystal before they have lost all their energy. The resolution on the pair peak of the 5.269 MeV y-ray is 8.3 keV, corresponding to a ?-ray energy resolution of 0.16 ~oThis is slightly better than the resolution that has been obtained with a high resolution magnetic Compton spectrometer2). Furthermore the germanium detector records the entire spectrum simultaneously. 2) H. T. Motz and R. E. Carter, Proceedings of I n t e r n a t i o n a l Conference on Nuclidic Masses, H a m i l t o n 1960; ed. by H. E. D u c k w o r t h ( U n i v e r s i t y of Toronto Press, 1960) p. 299.
HIGH
RESOLUTION
GAMMA-RAY" SPECTROMETERS
In fig. 3 we show a small region of the neutron capture ?-ray s p e c t r u m of iron. The doublet at 7.639 MeV with a separation of 14.4 keV is partially resolved. The resolution on a neighbouring single y-ray of 7.285 MeV was 9.8 keV, corresponding to an energy resolution of 0.13 %. The relative intensities of the 7.639 MeV doublet are(46 _+ 4) % to the ground state and (54 _+ 4) % to the 14.4 keV state.
185
from the surface of the 3.5 m m crystal. W i t h a thicker detector the effect of electron escape will be greatly reduced and for a detector 2 cm in d i a m e t e r b y 1 cm thick we estimate t h a t the efficiency for the double escape peak would be ~ 4 % at 7 MeV. The probability of absorption of the annihilation q u a n t a also increases and the single escape peak will become more prominent.
14.4 keV E (keY)
600
7639
Z
40C
t~.4 Fe 57
OC~ 200
-
7500
7600 Y-RAY
7700 ENERGY
7800
(keY)
Fig. 3. 7639 keV donblet observed in neutron capture gamma-ray spectrum of natural iron. The resolution (fwhrn)ort a neighbouring peak at 7285 keV is 9.8 keV correspondingto a gamma-ray energy resolution of 0.13~/o. The shape of this peak is used in resolving the components of the doublet as shown by the dashed lines. I n a detailed analysis of circular polarization experiments, Vervier 3) deduced a value of (49 + 19) % for the ground state transition. Recently Oroshev et al. 4) have partially resolved the doublet using a high resolution magnetic Compton spectrometer. We have estimated the intrinsic efficiency of the 3.5 m m depletion d e p t h detector b y comparing the counting rate in the double escape peak with t h a t observed simultaneously in a 3" × 3" NaI spectrometer, and correcting for solid angles and the efficiency of the NaI spectrometer. In this m a n n e r we obtain an efficiency of ~ 0.3 °/o at 2.614 MeV and ~ 0.2% at 7.639 MeV. The efficiency at 2.614 MeV is in reasonable agreement with the value calculated from the pair production cross-section and the probability of absorption of one of the annihilation quanta. The efficiency at 7.639 MeV is considerably lower t h a n the calculated value (1.8%) and indicates t h a t ~ 90}; of the high energy electrons have sufficient energy to escape
I n order to reduce the background below the "double escape peak" due to Compton scattering we have used the germanium lithiumdrift detector as the central crystal in a three-crystal pair spectrom e t e r with two NaI detectors observing the annihilation quanta. The arrangement is shown schematically in the inset to fig. 4. To illustrate the improvement which can be obtained, we show, in the upper half of fig. 4, the s p e c t r u m of Na 24 observed directly in the germanium crystal, and, in the lower half, the spectrum in coincidence with the two annihilation q u a n t a observed in the NaI scintillation spectrometers. The ratio of the peak height of the 2.75 MeV 7-ray to the height of background below the peak is i m p r o v e d from 7:1 to 150:1. At low energies, where the background is due to the Compton distribution of the 1.368 MeV y-ray the i m p r o v e m e n t is even greater. It is interesting to a) j. Vervier, Nuclear Phys. 26 (1961) 10. 4) L. ,,7. Groshev, A. M. Demidov, L. A. Kotelnikov and V. N. Lutsenko, Preprint (Moscow 1963).
186
G. T, E W A N
(o) DqRECT SPECTRUM
AND
7-Sll
I
I
SINGLE ESCAPE ~-27S4 2243
x,-JL~l
Y- 27S4 keV
keY
'
(b) SPECTRUM IN 3-CIRYSTAL SPECTROMETER
I u
o
TAVENDALE
1732 key
,.
0: w o_
J.
DOUBLE ESCA~ y 2754 yq368 keV
d aa Z
A.
1
I[~DOUBLE-ESCAPE~-2754 I
II
,O~ . . . .1.
J
I
\/i
0OUaLF-FSCA~
h°~
p. ~ D,ODE
"*>~ >.~:, :
! 3OO
i
,; ....
500
750
I000
1500 ENERGY
2000
2500
3000
(keV)
Fig. 4. Three-crystal pair spectrometer study of Na 2a using a lithium drift germanium crystal as the central crystal. Tile experimental arrangement (to scale) is shown at the right. The three-crystal arrangement greatly reduces the background below the pair production peak.
note t h a t the very weak 3.85 MeV y-ray 5) is observed both in the direct spectrum and the coincidence spectrum. The energy measured in our experiments is (3861 + 5) keV. In this letter we have discussed the use of germanium lithium-drift detectors for studying high energy ?-rays. We have also used lithium-drifted silicon detectors to study high energy },-rays6). The linear absorption coefficient for pair production of silicon is ~ 41 t h a t of germanium. Thus, for some experiments in which efficiency is not of prime importance, it is possible to use lithium-drift silicon
detectors which are at present more readily available. We would like to acknowledge the help of the T a n d e m accelerator group during our experiments on reaction ?-rays, and the assistance of Dr. G. A. Bartholomew in the experiments on neutron capture y-rays. We also wish to t h a n k Mr. I. L. Fowler and Dr. L. G. Elliott for helpful discussions. 5) L. V. Gustova, B. S. Dzhelepov,P. F. germolov and O. V. C h u b i n s k i i , l z v e s t . A k a d . N a u k . S S S R , Ser. F i z . 2 2 (1958) 21 I. 6) A. J . T a v e n d a l e , P a p e r t o b e p r e s e n t e d a t N i n t h S c i n t i l l a tion and Semi-Conductor Counter Symposium, Washington D . C . , F e b r u a r y 1964.
INSTRUMENTS
AND
METHODS
26
(1964)
183--186;
©
NORTH-HOLLAND
PUBLISHING
CO.
APPLICATION OF HIGH RESOLUTION LITHIUM-DRIFT GERMANIUM GAMMA-RAY SPECTROMETERS TO HIGH ENERGY GAMMA-RAYS G. T. E \ V A N a n d A. J. T A V E N D A L E
Chalk River Nuclear Laboratories, Atomic Energy o/Canada Limited Received 2 J a n u a r y 1964
The process is illustrated schematically in the inset to fig. 1 below. At a y-ray energy of 7.64 MeV we have achieved a resolution of 9.8 keV, corresponding to a y-ray energy resolution of 0.13 %. This is better
In a recent publication 1) we discussed briefly the use of germanium lithium-drift p-i-n diodes as high resolution y-ray spectrometers. W e report here the application of these detectors to the study of high DOUBLE
ESCAPE
Y-2614
1~92 key
-Z
3 keV
z io
7-26L4
ke~
w o_
o
,o'
iOZ[ I 1400
i
_
J
1600
..........
18100 .....
22:00
2000
2400
26100
ENERGY (keV) Fig. 1. H i g h energy region of g a m m a - r a y s p e c t r u m of Th(B -F C + C") observed with a l i t h i u m - d r i f t g e r m a n i u m detector. The detector was operated a t liquid n i t r o g e n t e m p e r a t u r e with a bias of 200 V. The spectrum of the 2614 keV g a m m a - r a y shows the intense double-escape peak, the weak single-escape peak, the full energy peak and the C o m p t o n d i s t r i b u t i o n . Tile origin of the double-escape peak is shown in the inset. I t is this peak t h a t we use in s t u d y i n g high energy gamma-rays.
energy y-rays. For this purpose we make use of the absorption of the y-rays by pair production and the detection of the total energy of the electron-positron pair in the crystal. This gives a peak at ( E - - 2roDc2) which for high energy y-rays is much more intense than the background due to Compton scattering. * N.R.C. Postdoctoral Felllow.
than has so far been obtained with either magnetic pair spectrometers or Compton spectrometers. The high resolution, combined with the simplicity of the detector, will make possible many new experii) A. J. T a v e n d a l e and G. T. Ewan, Nucl. Instr. and Meth., 25 1963) 185. iX. J. Tavendale, Proceedings of I n t e r n a t i o n a l S y m p o s i u m on Nuclear Electronics, Paris 1963, in press. 183
184
G. T. E W A N A N D A. J. T A V E N D A L E
merits with reaction and neutron capture ?-rays. To illustrate the type of spectrum obtained from a high energy ?-ray, we showin fig. 1 the spectrum observed from a radio-active source of Th (B + C + C"), using a detector 19 mm in diameter and with a depletion depth of 3.5 mm. The detector was operated at 77°K with a bias of 200 volts. The 2.614 MeV ?-ray has a peak corresponding to the full
k
5.297
from the double-escape peak to higher energies. For the crystal used here, this effect reduces the calculated intensity of the double-escape peak by a factor of ~ 0.6. We have used a detector 19 mm in diameter with a depletion depth of 3.5 mm to study ?-rays from reactions produced by the Chalk River Tandem accelerator, and neutron capture gamma-rays at MeV
N 14 ( d , p ) N '5
GAMMA-RAY MeV
I000
(3/2 -)
s 3 z l - -
1/2+ /' J bJ z ;;E
5.269 MeV
800
GAMMA-RAY
I
o
i a: w a_
600
8.5 keV
I o
_
:
I
I!
400
!
~• . . . .
i
5POO
"..%
• • . •. ,~ :.:..'~." :~-~-.,-~.4,,--;~..
v
I
,/~
a 15
Tantalum nitride target 4 MeV
deuterons
Germanium lithium drift p-i-n diode at liquid nitrogen temperature 19 mm diameter z 3-5 mm depletion depth
J
Gamma rays /Both annihilation ~ q ~ n t a escape
I
I
5200 T-RAY
5300 ENERGY
(keY)
Fig. 2. Region of g a m m a - r a y spectrum observed in the reaction N14(d, p) N 15 produced by 4 MeV deuterons. A schematic d i a g r a m of the e x p e r i m e n t a l a r r a n g e m e n t is shown a t the right. The g a m m a ray energy resolution is 0.16%.
energy, a Compton distribution, a weak singleescape peak and an intense peak due to pair production in which the electron and positron lose all their energy in the crystal, and the two annihilation quanta escape. It is this "double escape p e a k " which makes the detector very useful for studying high energy y-rays. The height of the peak is 8 times the height of the Compton distribution. For higher energy y-rays the pair production cross-section increases and the Compton cross-section decreases, so that the ratio of peak height to Compton distribution will improve. The resolution (fwhm) on the "double escape peak" is 7.3 keV and the measured intrinsic efficiency is ~ 0.3 %. This efficiency is less than would be calculated from the pair production cross-section because absorption or Compton scattering of the annihilation quanta will remove counts
the N R X reactor. In fig. 2 we show a small region of the N14(d,p)N15 y-ray spectrum. The doublet at 5.27 and 5.30 MeV is clearly resolved. Because of the relatively small depletion depth there is a tail on the low energy side of the lines due to escape of a large fraction of the electrons from the crystal before they have lost all their energy. The resolution on the pair peak of the 5.269 MeV y-ray is 8.3 keV, corresponding to a ?-ray energy resolution of 0.16 ~oThis is slightly better than the resolution that has been obtained with a high resolution magnetic Compton spectrometer2). Furthermore the germanium detector records the entire spectrum simultaneously. 2) H. T. Motz and R. E. Carter, Proceedings of I n t e r n a t i o n a l Conference on Nuclidic Masses, H a m i l t o n 1960; ed. by H. E. D u c k w o r t h ( U n i v e r s i t y of Toronto Press, 1960) p. 299.
HIGH
RESOLUTION
GAMMA-RAY" SPECTROMETERS
In fig. 3 we show a small region of the neutron capture ?-ray s p e c t r u m of iron. The doublet at 7.639 MeV with a separation of 14.4 keV is partially resolved. The resolution on a neighbouring single y-ray of 7.285 MeV was 9.8 keV, corresponding to an energy resolution of 0.13 %. The relative intensities of the 7.639 MeV doublet are(46 _+ 4) % to the ground state and (54 _+ 4) % to the 14.4 keV state.
185
from the surface of the 3.5 m m crystal. W i t h a thicker detector the effect of electron escape will be greatly reduced and for a detector 2 cm in d i a m e t e r b y 1 cm thick we estimate t h a t the efficiency for the double escape peak would be ~ 4 % at 7 MeV. The probability of absorption of the annihilation q u a n t a also increases and the single escape peak will become more prominent.
14.4 keV E (keY)
600
7639
Z
40C
t~.4 Fe 57
OC~ 200
-
7500
7600 Y-RAY
7700 ENERGY
7800
(keY)
Fig. 3. 7639 keV donblet observed in neutron capture gamma-ray spectrum of natural iron. The resolution (fwhrn)ort a neighbouring peak at 7285 keV is 9.8 keV correspondingto a gamma-ray energy resolution of 0.13~/o. The shape of this peak is used in resolving the components of the doublet as shown by the dashed lines. I n a detailed analysis of circular polarization experiments, Vervier 3) deduced a value of (49 + 19) % for the ground state transition. Recently Oroshev et al. 4) have partially resolved the doublet using a high resolution magnetic Compton spectrometer. We have estimated the intrinsic efficiency of the 3.5 m m depletion d e p t h detector b y comparing the counting rate in the double escape peak with t h a t observed simultaneously in a 3" × 3" NaI spectrometer, and correcting for solid angles and the efficiency of the NaI spectrometer. In this m a n n e r we obtain an efficiency of ~ 0.3 °/o at 2.614 MeV and ~ 0.2% at 7.639 MeV. The efficiency at 2.614 MeV is in reasonable agreement with the value calculated from the pair production cross-section and the probability of absorption of one of the annihilation quanta. The efficiency at 7.639 MeV is considerably lower t h a n the calculated value (1.8%) and indicates t h a t ~ 90}; of the high energy electrons have sufficient energy to escape
I n order to reduce the background below the "double escape peak" due to Compton scattering we have used the germanium lithiumdrift detector as the central crystal in a three-crystal pair spectrom e t e r with two NaI detectors observing the annihilation quanta. The arrangement is shown schematically in the inset to fig. 4. To illustrate the improvement which can be obtained, we show, in the upper half of fig. 4, the s p e c t r u m of Na 24 observed directly in the germanium crystal, and, in the lower half, the spectrum in coincidence with the two annihilation q u a n t a observed in the NaI scintillation spectrometers. The ratio of the peak height of the 2.75 MeV 7-ray to the height of background below the peak is i m p r o v e d from 7:1 to 150:1. At low energies, where the background is due to the Compton distribution of the 1.368 MeV y-ray the i m p r o v e m e n t is even greater. It is interesting to a) j. Vervier, Nuclear Phys. 26 (1961) 10. 4) L. ,,7. Groshev, A. M. Demidov, L. A. Kotelnikov and V. N. Lutsenko, Preprint (Moscow 1963).
186
G. T, E W A N
(o) DqRECT SPECTRUM
AND
7-Sll
I
I
SINGLE ESCAPE ~-27S4 2243
x,-JL~l
Y- 27S4 keV
keY
'
(b) SPECTRUM IN 3-CIRYSTAL SPECTROMETER
I u
o
TAVENDALE
1732 key
,.
0: w o_
J.
DOUBLE ESCA~ y 2754 yq368 keV
d aa Z
A.
1
I[~DOUBLE-ESCAPE~-2754 I
II
,O~ . . . .1.
J
I
\/i
0OUaLF-FSCA~
h°~
p. ~ D,ODE
"*>~ >.~:, :
! 3OO
i
,; ....
500
750
I000
1500 ENERGY
2000
2500
3000
(keV)
Fig. 4. Three-crystal pair spectrometer study of Na 2a using a lithium drift germanium crystal as the central crystal. Tile experimental arrangement (to scale) is shown at the right. The three-crystal arrangement greatly reduces the background below the pair production peak.
note t h a t the very weak 3.85 MeV y-ray 5) is observed both in the direct spectrum and the coincidence spectrum. The energy measured in our experiments is (3861 + 5) keV. In this letter we have discussed the use of germanium lithium-drift detectors for studying high energy ?-rays. We have also used lithium-drifted silicon detectors to study high energy },-rays6). The linear absorption coefficient for pair production of silicon is ~ 41 t h a t of germanium. Thus, for some experiments in which efficiency is not of prime importance, it is possible to use lithium-drift silicon
detectors which are at present more readily available. We would like to acknowledge the help of the T a n d e m accelerator group during our experiments on reaction ?-rays, and the assistance of Dr. G. A. Bartholomew in the experiments on neutron capture y-rays. We also wish to t h a n k Mr. I. L. Fowler and Dr. L. G. Elliott for helpful discussions. 5) L. V. Gustova, B. S. Dzhelepov,P. F. germolov and O. V. C h u b i n s k i i , l z v e s t . A k a d . N a u k . S S S R , Ser. F i z . 2 2 (1958) 21 I. 6) A. J . T a v e n d a l e , P a p e r t o b e p r e s e n t e d a t N i n t h S c i n t i l l a tion and Semi-Conductor Counter Symposium, Washington D . C . , F e b r u a r y 1964.